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Former "Good Morning America" co-host Joan Lunden revealed on Tuesday that she has been diagnosed with breast cancer.

Posting to fans via Twitter and a letter on her website JoanLunden.com, Lunden said she had received an all-clear two weeks ago after her annual mammogram, but "for women who have dense fibrous breast tissue, as I do, often our doctors will recommend an ultrasound as well."

"My ultrasound that day revealed a tumor in my right breast," she wrote. "After a core biopsy was performed, I heard those words that every woman fears and that I never thought I'd hear: 'you have breast cancer.' "

Lunden, who co-hosted "GMA" from 1980 to 1997, said she initially considered keeping the news private, but decided to go public with her diagnosis.

"I have decided to talk about my breast cancer because since the moment I took the job at 'Good Morning America' I have lived my life sharing my joys and my disappointments with all of you: my pregnancies, my relationships, my career," she said. "I have shared my journey. So it certainly didn't feel right keeping this part of my journey a secret."

Celebrities battle cancer 30 photos

Celebrities battle cancer30 photos

Def Leppard guitarist Vivian Campbell, 52, is pulling out of the band's tour after his cancer Hodgkin lymphoma returned.

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Celebrities battle cancer30 photos

Tommy Chong of Cheech & Chong, who was diagnosed with prostate cancer in 2012, told Us magazine that he is undergoing treatment for rectal cancer. As he did for the prostate cancer, he's using marijuana to take the edge off: "I'm using cannabis like crazy now, more so than ever before," he told the magazine.

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Celebrities battle cancer30 photos

Sir John Hurt, known for performances in "Alien," "The Elephant Man" and the Harry Potter movies, told the British Press Association that he has pancreatic cancer. The disease is in its early stages, he said, and he is "more than optimistic about a satisfactory outcome, as indeed is the medical team."

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Celebrities battle cancer30 photos

TV chef and author Sandra Lee revealed in May that she has breast cancer. Her longtime boyfriend, New York Gov. Andrew Cuomo, announced that he would be taking some personal time to support her through her double mastectomy.

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Celebrities battle cancer30 photos

Actress Rita Wilson, who can be seen on HBO's "Girls," revealed April 14 that she is fighting breast cancer and has undergone a double mastectomy. She thanked her family, including husband Tom Hanks, and doctors for their support in a statement to People magazine.

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Celebrities battle cancer30 photos

Neurologist Dr. Oliver Sacks, perhaps best known as the inspiration for the doctor portrayed by the late actor Robin Williams in the 1990 film "Awakenings," revealed in February that he has cancer. It's not Sacks' first bout with the disease. Nearly a decade ago, he was diagnosed with ocular melanoma that eventually left him blind in one eye and served as the basis for one of his books, "The Mind's Eye."

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Celebrities battle cancer30 photos

Fans of singer Morrissey knew the star had been ill after he canceled some U.S. tour stops in June, but it appears the performer has been battling cancer. "They have scraped cancerous tissues four times already, but whatever," Morrissey said in an interview with Spanish-language outlet El Mundo. "I am aware that in some of my recent photos I look somewhat unhealthy, but that's what illness can do. I'm not going to worry about that."

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Diem Brown, one of the stars of MTV's "The Challenge," was diagnosed with cancer for the third time in August. Doctors discovered a tumor blocking her kidneys, People.com reported, slowing her recovery. Brown died in November.

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Celebrities battle cancer30 photos

When former "Good Morning America" host Joan Lunden learned she was facing an "aggressive" form of breast cancer, she was determined to face her health battle head on. Knowing she would need chemotherapy, Lunden decided to remove her familiar blond hair before her locks could be affected by the treatment. "You know it's going to happen one of these days and you are wondering how or when," Lunden explained to People magazine, which she posed for without her wig in September. "So I just owned it."

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Celebrities battle cancer30 photos

Hugh Jackman took to Instagram in May to share a photo of his nose, which was bandaged because of treatment he had on basal cell carcinoma.

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Former "Dancing With the Stars" co-host Samantha Harris was diagnosed with breast cancer and underwent a double mastectomy. Harris told People magazine she detected a lump during a self-exam and then followed up with a specialist after receiving a clean mammogram.

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Celebrities battle cancer30 photos

Actress Brittany Daniel of "Sweet Valley High" and "The Game" has been quietly dealing with stage IV non-Hodgkin's lymphoma. Daniel recalls in an issue of People magazine that her 2011 diagnosis "happened so suddenly," but she's been able to face it with the support of her family.

ABC's Amy Robach found out she had breast cancer in November 2013 after she had a mammogram done live on "Good Morning America" for cancer awareness month.

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Radio personality Robin Quivers quietly battled cancer for months, but she had happy news to share with "Howard Stern" listeners in September 2013. On the show, Quivers revealed that her doctors believe she's cancer-free after receiving treatment, including chemotherapy.

Sharon Jones held off plans to tour and release a new album with the Dap-Kings after being diagnosed with stage 1 bile duct cancer in 2013.

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Michael Douglas offered some interesting insight as to how he may have developed the throat cancer that he was diagnosed with in August 2010. Douglas later told the "Today" show that his tumor was gone.

Actress Christina Applegate had a bilateral mastectomy in 2008. Doctors had diagnosed her with cancer in her left breast and offered her the options of either radiation treatment and testing for the rest of her life or removal of both breasts.

KISS band member Peter Criss sat down with CNN's Elizabeth Cohen in 2009, a year after his battle with breast cancer. The musician said he wanted to increase awareness of the fact that men can also get the disease.

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Cynthia Nixon not only joined the cast of Showtime's "The Big C," about a woman battling the disease, and portrayed a woman with cancer in the Broadway play "Wit" -- Nixon was diagnosed with breast cancer in 2006.

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Zoraida Sambolin was diagnosed with breast cancer in April 2013, and she chose to have a double mastectomy. Sambolin said that Angelina Jolie's New York Times opinion piece about undergoing the procedure gave her courage to share her story.

"Three's Company" star Suzanne Somers spoke with CNN's Piers Morgan in 2012 about her stem cell surgery and her bout with breast cancer. She was diagnosed in 2001, which is when she began researching alternative methods to reconstructive surgery.

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Olivia Newton-John was diagnosed in 1992, and the singer has become an advocate for breast self-examination.

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Lunden turned to current "GMA" co-host and breast cancer survivor Robin Roberts to offer her first TV interview about having the disease. The news hit home for the staff, as "GMA" co-host Amy Robach is currently being treated for breast cancer.

Lunden told Roberts that the form of cancer she has is particularly aggressive.

"In the beginning it's such a shock, it's a stunner," Lunden told Roberts. "For me, I'm a health advocate, I'm all over America talking to women saying, 'Make sure you get all of your checkups.' In the beginning you almost feel like 'What did I do wrong?'"

Lunden, whose late father was a cancer surgeon, said she gets mammograms every year. She is now in "warrior mode" learning everything she can about the disease and is currently undergoing chemotherapy, she said.